Gating of voltage-dependent potassium channels.
Activation of voltage-dependent ion channels is primarily controlled by the applied potential difference across the membrane. For potassium channels the Drosophila Shaker channel has served as an archetype of all other potassium channels in studies of activation mechanisms. In the Shaker potassium channel much of the voltage sensitivity is conferred by the S4 transmembrane helix, which contains seven positively charged residues. During gating, the movement of these charges produces gating currents. Mutagenic and fluorescence studies indicate that four of these residues are particularly important and contribute to the majority of gating charge, R362, R365, R368 and R371. The channel is thought to dwell in several closed states prior to opening. Ionic-charge pairing with negatively charged residues in the S2 and S3 helices is thought to be important in regulating these closed states and detailed kinetic models have attempted to define the kinetics and charge of the transitions between these states. Neutral residues throughout the S4 and S5 helices are thought to control late steps in channel opening and may have important roles in modulating the stability of the open state and late closed states. In response to depolarization, the S4 helix is thought to undergo a rotational translation and this movement is also important in studies of the movement of the pore helices, S5 and S6, during opening. This review will examine residues that are important during activation as well as kinetic models that have attempted to quantitatively define the activation pathway of voltage-dependent potassium channels.